1. Field of the Invention
This invention relates to gas turbine engine compressor bleed and, more particularly, to bleed ports in the compressor for extracting two or more portions of compressor air from a single stage of the compressor.
2. Discussion of the Background Art
Gas turbine engines, such as a bypass turbofan engine, bleed or extract air between stages of a multi-stage axial compressor for various purposes. The extracted air is often referred to as secondary air. Secondary air is usually required for turbine cooling, hot cavity purging or turbine clearance control and is often referred to as domestic bleed because it is used for the engine. Secondary air is also often required to pressurize the aircraft cabin and for other aircraft purposes and, is thus, referred to as customer bleed. Domestic bleed flow levels are generally a constant percentage of compressor flow (i.e. 2%), whereas customer bleed requirements typically vary (i.e. 0-10%).
It is frequently desirable to have both customer and domestic bleed extracted from the same stage of the compressor, where the air has the desired pressure and temperature properties. This is, typically, desirable in a gas turbine engine having a low number of stages in the high pressure ratio compressor. The problem that this poses is to design a bleed system that allows the customer bleed to be modulated with minimal impact on the bleed pressure supplied to domestic bleed. If the domestic bleed pressure is allowed to drop below a threshold level, then, insufficient cooling air may be supplied to the hot section of the engine, resulting in decreased life on hot parts.
Conventional engines are designed with the customer and the domestic bleed ports isolated at different stages of the compressor and, thus, the domestic bleed pressure is relatively insensitive to the customer bleed rate. A high recovery bleed slot to supply both the customer and domestic bleeds has been used in engines with a low number of high pressure compressor stages. The problem with two bleed circuits using the same slot and plenum is that the slot recovery and, hence, the plenum pressure is very sensitive to the level of customer bleed.
At high levels of customer bleed, the bleed slot throat and exit Mach numbers become high and large dump losses are realized at the slot exit into the plenum. This significantly reduces the pressure available to the domestic bleed circuit. It is, thus, highly desirable to have a means for bleeding air from a compressor for two or more different air circuits, such as the customer and domestic bleeds, and being able to modulate one of the circuits with minimal impact on the bleed pressure supplied to the bleed for the other circuit or circuits.